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Discussion > Simon Abingdon/Jonathan Jones/Radiative transfer

Apr 11, 2012 at 10:24 PM Jonathan Jones
(Prévost)...Consider me unimpressed.

Apr 11, 2012 at 10:42 PM Jonathan Jones
What makes @mdgnn and his supporters unusual (beyond their fondness for caloric) is the claim that these effects persist over arbitrarily large distances.

I'm grateful for the detailed discussion. It has (I think) helped me grasp what mdgnn has been on about.

I had always understood that a photon was a pretty dumb entity, that retained no memory of what temperature of the body that emitted it (except that its energy/wavelenght gave a rough indication of what that might have been) and also, a photon meeting a black body had no option but to be absorbed, irrespective of the temperature of the black body).

For me, this discussion has confirmed my understanding of physics was correct:

- A body radiates power proportional to T^4.

- A photon, radiated by a body at any temperature T1 whatever, that impacts a black body at any temperature T2 whatever, is absorbed by the black body.

So far as I can see, from these two principles, you can work out what happens if a heated body, instead of radiating directly into empty (0K) space, has to heat another intermediate body (eg an enclosing shell), which then radiates into empty space. Reiterating your discussion...

In outline, what happens is:
- the intermediate body radiates both to empty space and back to the heated body.
- so the original body now has to be hotter to get rid of the power that was heating it.

The radiation from the intermediate body back to the original body is the "back radiation" that some commenters to blogs state cannot exist and defies the laws of thermodynamics. The presence of this "back radiation" has caused the original body to be hotter than it would be without the intermediate body. But there is nothing contrary to the laws of thermodynamics. The original body has been heated to a higher temperature by its heater, not by the "back radiation".

Thank you for confirming the physics that the 20th century depended on still makes sense.

Apr 12, 2012 at 9:33 AM | Registered CommenterMartin A

It's just a gut feeling. Most machines which involve temperature and radiating heat (e.g. combustion engines, chemical processors, supersonic aircraft) have been modelled and tested assuming the radiative properties of materials in contact and separated by air are as described by classical thermodynamic theory.

If that theory is plain wrong in the fundamental way described by mdgnn, I just feel that the tests and models carried out on all these numerous machines would have shown it up by now, and these machines and processes would have gone out of their designed engineering limits.

I have no proof of this, obviously.

Apr 12, 2012 at 9:54 AM | Unregistered CommenterTheBigYinJames

Martin A puts it well. The result flows simply from the properties of black bodies: if you posit the existence of perfect absorbers that also act as perfect emitters with a total radiated power proportional to T^4 then everything can be derived just by solving for the steady state.

Essentially the claim by @mdgnn seems to be that black bodies cease to behave as black bodies once they are radiatively coupled; he decorates his claim with lots of confused references to Prévost, but that's not critical to trying to understand what he is saying.

As The BigYinJames says actually doing the experiment would be quite difficult: the situation has been idealised to make the theory simple, which comes at the cost of making the experiment hard! But it could be done reasonably well if needed. Nobody bothers because the two disc result is just a trivial consequence of more fundamental properties (the behaviour of black bodies) which have been checked in great detail.

Apr 12, 2012 at 10:19 AM | Registered CommenterJonathan Jones

Apr 12, 2012 at 10:07 AM TheBigYinJames

Dr Spencer did a good page on this:

Yes, and "former NASA scientist Pierre R Latour, PE, PhD Chemical Process Control Systems Engineer" then came up with a webpage titled No Virginia Cooler Objects Cannot Make Warmer Objects Even Warmer Still.

This webpage says things like "What actually happens is the chiller radiates to the hot plate, but the plate cannot absorb any of it because it is too cold. The hot plate reflects, transmits or scatters colder radiation, just like my roof does for cold radio waves." Many sentences, such as this one, contain several nonsenses simultaneously.

I think that Dr Latour is associated with the authors of "Slaying the Sky Dragon - Death of the Greenhouse Gas Theory".

It's a shame that such a book has been published. You can't debunk faulty alarmism science with gobbledegook science. It makes it possible for CAGW alarmists to say "There you are: skeptics talk nonesense".

Apr 12, 2012 at 1:03 PM | Registered CommenterMartin A

Martin A:

It's a shame that such a book has been published. You can't debunk faulty alarmism science with gobbledegook science. It makes it possible for CAGW alarmists to say "There you are: skeptics talk nonesense".

Thanks for saying this Martin - and for the other things further up, and for pursuing mdgnn for many moons across many threads on these matters.

As I was retracing my steps on pseudonymity this week I came across this from Leo Hickman in June last year:

I admit that I sometimes find it hard to detect the signal from all the noise when observing climate sceptics.

Now Leo is not a match for Jonathan on the physics, of that I think we can be sure, but he is influential in the wider debate, like or not. I believe that there are some very unscrupulous operators in the climate area who greatly wish for people like Hickman to continue to 'find it hard to detect the signal' from climate sceptics. There's a price for indulging the likes of mdgnn obsessing on any page of BH he chooses. This plays into more general concerns I have but the specific example is extremely important in its own right, especially if you take into account all the 'slayers'. Very well said.

Apr 13, 2012 at 8:29 AM | Unregistered CommenterRichard Drake

@ Jonathan Jones

Thank you for this!

Apr 13, 2012 at 8:44 AM | Unregistered CommenterDouglas J. Keenan

This was a useful thread! I see that mdgnn was asking about where it was, but otherwise is pretty much priding himself on the 'real world' nature of his vision, as against the abstruse 'two plates in vacuum' discussion. Well, you can lead a horse to water...

I agree with the discussion above, where the implication is that his different vision may be due to assuming 'strong coupling' between the two plates. But I wonder if he is not also confusing the present problem, where there is a steady state, with a continuous net flow of heat from the heater to plate 1 to plate 2 to the outer darkness, with an equilibrium state. If you make plate 2 fully isolated on the 'outer' side, give a burst of heat to the system, then turn off the heater, then assuming there are no losses, the two plates will exchange heat until they have the same temperature (if they have the same heat capacity, the average of their two initial temperatures) and thereafter, the IR photons flowing in one direction will be exactly matched by the IR photons flowing in the other direction. In the limit where the amount of heat flowing into plate 1, and out to space from plate 2, is small compared to the amount of heat being exchanged between the two (imagine plate 2 is almost completely isolated on the outside, with just a tiny emissive patch), you will be close to equilibrium, so the two plates will have the same temperature, near enough (equally, this is the strong coupling limit).

Finally, the analogy to the atmosphere has been deliberately soft-pedalled here, which is a good thing as it enabled a focused discussion. Also, it should be realized that the analogy is not all that good. It is true that much of the heat applied to earth is due to incoming visible light hitting the surface (plate 1), and that most of the heat is lost by black-body type radiation from regions fairly high in the atmosphere (plate 2). However, heat exchange between the surface and the atmosphere is not only by radiation - the earth is not in a vacuum, and air can move up and down convectively, and carry heat with it, in the form of heat itself and in the form of water vapour (which, upon condensing higher up to form rain releases lots of heat). This complicated things quite a bit. Also, the earth is a geographically variegated sphere, not a perfect plate, and it is only heated from one side, though it rotates, etc etc. So it is not possible to make any kind of 'back of the envelope' calculation of the precise temperature on earth. That's why you need models, which are basically hugely extended versions of Jonathan's system, which need to be executed on a computer rather than with paper and pencil. They sort of work OK (they tend to get the temperature in the right ballpark), but they are very far from perfect.

Apr 13, 2012 at 9:18 AM | Registered CommenterJeremy Harvey

Richard, thank you.

Early this year I posted a review of Slaying the Sky Dragon on

It's currently rated as "the most helpful critical review".

A surprising thing to me is not that many people don't understand basic physics. What is surprising to me is how many don't understand basic physics but have convinced themselves and many others that they do.

Apr 13, 2012 at 9:33 AM | Registered CommenterMartin A

Jeremy Harvey is (as usual) quite right.

I too had wondered whether mdgnn was thinking about alternative versions of the steady state; that's one reason why I have been trying to find cases where he makes an unambiguous prediction of the behaviour of a clearly defined situation. Unfortunately these are very hard to find in his writings, despite their quite impressive extent.

It is of course entirely true that this model is massively oversimplified, and you need to go a great deal further to establish how much (if any of) this survives in a system of variably heated grey bodies with convection, not to mention the host of other factors that complicate the real earth. (Nullius in verba makes this point quite well on one of the main threads.) That's a far more complex question, and not something I am competent to have a detailed opinion on. My point here is simply to establish what back radiation is and why it exists in the idealised situation discussed above.

Apr 13, 2012 at 9:47 AM | Registered CommenterJonathan Jones

Seems to be that the 2nd Law's "simplified" wordage, i.e. "a cold body can't heat up a warm body" is a little bit misleading if you don't look carefully - the actual law has a bit on the end... "without using work", which is the clincher here. The internal heater is providing the extra work.

Now if only mdgnn would come here and explain how his theory changes this.

Apr 13, 2012 at 9:58 AM | Unregistered CommenterTheBigYinJames

The BigYinJames,

It is notoriously difficult to state the 2nd law correctly, and easy to get confused by naive statements of it.

Apr 13, 2012 at 10:33 AM | Registered CommenterJonathan Jones

So now mdgnn is saying that the fact that the differing energy flows are at different frequencies alters these balances.... and I'll have to hit the books to find out why/not. This thread has only been talking about net energy flows, not differing rates for differing frequency of radiation.


Apr 13, 2012 at 2:12 PM | Unregistered CommenterTheBigYinJames

TheBigYinJames, mdgnn is again incorrect, I think. The wavelength of emission from the bodies is irrelevant for the problem as posed here. For what it is worth, though, the wavelength distribution of emitted radiation from a perfect black body is well known. It is given by the Planck Formula, which I will not attempt to type here.

Apr 13, 2012 at 2:23 PM | Registered CommenterJeremy Harvey


Mdgnn seems to be worrying about the timescale for thermalisation; in particular he may be making a distinction between a "black body" and a "black body at thermal equilibrium". When Jeremy and I have been using phrases such as "perfect black body" or "ideal black body" we have been meaning "black body at thermal equilibrium". This is reasonably standard terminology but it is a trifle loose.

Once again, however, this is largely relevant to the question of whether a set of black bodies in thermal equilibrium is in any way a sensible model of the earth/atmosphere system rather than the behaviour of ideal black bodies, which by definition themalise incoming radiation.

Apr 13, 2012 at 3:24 PM | Registered CommenterJonathan Jones

Jonathan, did you mean largely irrelevant there?

Apr 13, 2012 at 3:34 PM | Unregistered CommenterRichard Drake

Yes, I know it's dangerous to extrapolate from ideal models designed to explain a concept to a real-world system as complex as atmospheric physics. Dangerous and stupid, probably.

If the best brains in the world haven't cracked it, I'm not going to manage it with a weekend browse of my old dog-eared undergraduate copy of Sears, Zemansky & Young.

As a 20 year old, I was able to prove Schrödinger's Time-Independent Wave Equation from first principles, so I should, even as an oldster, be able to grasp the concepts of what mdgnn is on about. I'm going to try, anyway.

Apr 13, 2012 at 3:35 PM | Unregistered CommenterTheBigYinJames

Richard Drake, I think I meant what I said, although the phrasing might be unclear.

Arguments about thermalisation cannot be deployed to question whether idealised black bodies behave as we have discussed, because by definition idealised black bodies thermalise radiation. They can be deployed to question whether a supposed ideal black body actually is one. Thus the question of thermalisation is irrelevant to the topic of this thread but relevant to the question of whether this thread has any correspondence with reality.

Apr 13, 2012 at 3:53 PM | Registered CommenterJonathan Jones

Jonathan, you are right, mdgnn seems to have an idea about thermalisation. That may be what TBYJ is referring to earlier. Well, I can't see myself that this is relevant, but thought I'd try to work out what might be going on.

I think what mdgnn is talking about concerns the processes that can happen to a CO2 molecule, high in the atmosphere. For the purposes here, I think there are four important ones:

(a) absorbtion of a photon of IR light, thereby moving CO2 from its ground state to its first vibrationally excited state (let us assume here that this is the only vibrational state we need to worry about), in which it undergoes bending motions. Lets call this CO2*, and write the absorption as:

CO2 + hf --> CO2*

(b) collisional excitation, whereby a CO2 molecule collides with another molecule, and in the process converts into CO2* (the required energy comes from the translational energy of the molecules - they move a bit slower after the process than before - they have effectively slightly cooled down).

CO2 + M --> CO2* + M

(c) collisional cooling, the reverse of (b):

CO2* + M --> CO2 + M + energy

In collisional cooling, the molecule bumps into another one, say of nitrogen (M = N2), and relaxes to its ground state. The energy released is not converted into a photon, but into translational energy of the molecules - by moving a bit faster, they have effectively warmed up a bit.

(d) spontaneous emission, the reverse of the absorption process (a) above. CO2* loses a photon. This can occur in a random direction, e.g. upwards or downwards.

At the top of the atmosphere, the sequence of steps (b) then (d) allows energy to be emitted into space. As described eloquently by Nullius in Verba on another recent thread, the heat gets to the top of the atmosphere through convection processes, largely.

I think mdgnn is talking about the relative rate of processes (c) and (d). If (c) is faster, then you get an equilibrium between processes (b) and (c), such that the populations of CO2 and CO2* are defined by the local temperature. The gas is thermalised. If however (d) is faster than (c), then upon formation of CO2* by process (a), due to a photon coming from lower down in the atmosphere, then the photon could be re-emitted (presumably upwards only in mdgnn-land - in reality it could occur in any direction) before collisional cooling (c), and the system would not be thermalized. I suppose this could be not accounted for in climate models.

I've tried to estimate the ratio of the rates of these two processes, and for this, I've used data I found in this review on CO2 properties, which I assume to be reliable for my present purposes.

The rate for the 'reaction' CO2* + N2 --> CO2 + N2 (Ns is the main collision partner) is given in Fig. 11, p. 18 of the document. At 0 C, right-hand side of the graph, k is ca. 10^-15 cm^3 s^-1). I estimate that the concentration of N2 molecules in the upper atmosphere is about 10^19 per cubic centimeter. So that would yield a relaxation rate for (c) of about 10^4 s^-1 - most molecules of CO2* would be cooled in less than a millisecond.

Spontaneous emission (d) from the lowest excited vibrational state of CO2 is shown in that document to occur with a rate constant (Einstein coefficient) of 1.6 s-1 (p. 33, Table 5, entry 7). I.e. emission would only occur on a timescale of about a second - much slower than (c).

OK, there are lots of other numbers in there, but most of the processes similar to (d) do seem to have slow rate coefficients, whereas vibrational cooling similar to (c) is quite fast. This is actually an area related to some of the research I do, and my back-of-the-envelope estimate here fits with what I expected: for small molecules, cooling (c) is pretty fast at most pressures. So I think we can assume that photons absorbed by CO2 do get thermalised.

Note that in a metal, the equivalent of process (c) is even faster, so it usually is a pretty good approximation to assume that solids acting as black bodies do get thermalized.

Apr 13, 2012 at 4:29 PM | Registered CommenterJeremy Harvey


Interesting. I suspect you would also have to worry about transfers of the excitation to other ro-vibrational states in CO2 through anharmonicities in the potential. A paper by Simpson caught my eye (he was one of my lecturers, so it's nice to finally learn what he was doing with his shock tubes)

The vibrational deactivation of the bending mode of CO2 by O2 and by N2, Chem Phys Lett 47, 133 (1977) ($$)

where he discusses this.

Clearly it's all very complicated but was heavily studied in the 1970s, driven both by the design of CO2 lasers and by early climate work. I think we can safely assume that the issues are well understood.

Apr 13, 2012 at 5:02 PM | Registered CommenterJonathan Jones

This ‘backradiation’ nonsense is fun to play with as a mental exercise but, in order to take account of the emissivity and atmospheric density of CO2, if an area of the surface of the Earth is to be the heated radiating plate, the ‘backradiating’ CO2 must be represented by a hemisphere [ to allow for scattering ] with a surface area of approximately 0.0004 of the radiating surface area.

Lets get real, as my nurse once said, “what’s all the fuss over such a little thing?”

Apr 14, 2012 at 3:47 AM | Unregistered CommenterRKS

We can all talk about our little things but I'm not sure that's what Jonathan had in mind by a 'private' thread :)

Apr 14, 2012 at 9:54 AM | Unregistered CommenterRichard Drake

We can all talk about our little things but I'm not sure that's what Jonathan had in mind by a 'private' thread :)

Apr 14, 2012 at 9:54 AM | Richard Drake>>>>>>>>>>>

How many contributing to this 'private thread' so far?

My contribution was definitely 'on thread', I just wondered why so much time and effort is spent doing the IPCC's work by discussing the finer points of such an insignificant contributor to climate.

Apr 14, 2012 at 10:26 AM | Unregistered CommenterRKS

I was actually a joke RKS. Sorry about that. But I'll offer an opinion anyway. It's misleading and tendentious to say people are "doing the IPCC's work" here. The idea is to get at the truth. If the IPCC happens to have got close to the truth in one area, so be it. Richard Lindzen for example has always said that the bulk of WG1 is pretty good, if you ignore the summary for policy makers. He wouldn't I'm sure include the hockey stick graph in 2001.

You are it seems to me trying very hard to reinstate a false alternative: either one has to believe the IPCC and its extremist acolytes on every bit of science and scarification being pushed or hey presto, mydogsgotnonose is right about radiative transfer. Good game, good game, as Bruce Forsyth might say, but it's time to advance a bit from that very low point. Thanks to Jonathan, Jeremy and others who have done an excellent job of doing so.

Apr 14, 2012 at 11:10 AM | Unregistered CommenterRichard Drake

You are it seems to me trying very hard to reinstate a false alternative: either one has to believe the IPCC and its extremist acolytes on every bit of science and scarification being pushed or hey presto, mydogsgotnonose is right about radiative transfer. Good game, good game, as Bruce Forsyth might say, but it's time to advance a bit from that very low point. Thanks to Jonathan, Jeremy and others who have done an excellent job of doing so.

Apr 14, 2012 at 11:10 AM | Richard Drake>>>>>>>>>>

Thanks for your response. I only realized the joke in your post after I'd posted my reply.

I am one of an increasing number of people who believe the real world physics of gas laws and insolation as being the 99% + [virtually sole] cause of mean global temperature, by means of gravity induced atmospheric pressure and solar radiation.

From that point of view, the empirically unsupported hypothesis of CO2 being any other than an almost insignificant contributor, via 'back radiation' to global temperature is the "false alternative"

When I see Prevost exchange being ridiculed because it was proposed in 1791, apparently no longer 'trendy' because it is somehow past some imaginary 'sell by' date, and the evidence of the statistical lack of global warming for the past 15 years [other than the el nino blip in 1998], accompanied by rapid rises in atmospheric CO2, I think it's about time that people called a halt to the almost religious navel gazing at the radiative properties of CO2, and spent a lot more effort studying real word climate science.

It's all about taking a 'third person' approach towards gaining a sense of perspective before deciding where to best apply one's skills.


Apr 14, 2012 at 5:32 PM | Unregistered CommenterRKS